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Astron. Astrophys. 347, 169-177 (1999)

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5. Conclusions

5.1. Imaging

Our deep images show that the PN are generally much larger than previously measured. The ratio of circular : non-circular PN in our sample is 6 : 3 and that of circular PN with/without central hole is 3 : 3. We have one object with prominent ansae (PN G253.5+10.7). Four PN exhibit outer shells: one prominent shell with halo (PN G231.8+04.1), one with extensions, outer loops, etc. (PN G257.5+00.6), and two with distinct steps in surface brightness near their outer edge (PN G214.9+07.8, PN G293.6+10.9).

It appears worth doing further very deep imaging of PN in order to study their morphology in detail, the ejection mechanism of the nebula matter, and their interaction with the ISM, e.g. likely in the case of PN G231.8+04.1 and PN G257.7+00.6. The knowledge about the spatial distribution of nebula matter will allow to construct consistent models PN[FORMULA]CSPN (Rauch ea. 1994, 1996), especially if a deprojection technique (Bremer 1995) can be applied in addition. These (3-dimensional) models will then be an important input for investigations dealing with properties of the ISM (e.g. Soker & Dgani 1997, Dgani & Soker 1998).

5.2. Spectroscopy

Useful spectra were obtained for four CSPN which are all H-rich (Table 4): two have approximately a solar He/H ratio (PN G214.9+07.8, PN G283.6+25.3), two have approximately twice the solar He/H ratio, (PN G231.8+04.1, PN G293.6+10.9).

The centrally located objects of PN G257.5+00.6 and PN G277.1-03.8 are probably late-type companions outshining the hot exciting stars in the optical spectra. UV spectra are required in order to identify the hot stars and to derive their photospheric parameters. These stars appear to be Abell 35-like objects (Jasniewicz & Acker 1993, Gatti et al. 1997). The presence of absorption features which might arise from a cool companion (Fig. 12) makes PN G283.6+25.3 another hot candidate for such an object. Presently, only three of those are known, namely Abell 35, LoTr 1, and LoTr 5. The discovery of two or three (?) more of these objects alone is remarkable and follow-up spectroscopy, high-resolution optical as well as ultraviolet, is highly desirable to investigate their cool and hot components, respectively.

5.3. H-rich / He-rich CSPN

The analysis revealed that our selection criteria are indeed able to pick out hot CSPN: all CSPN, which could be analyzed, are hotter than the typical stars from the Napiwotzki & Schönberner (1995) sample. Their parameter range ([FORMULA] and [FORMULA]) is identical to the range preferentially populated by PG 1159 stars (cf. Dreizler et al. 1995), but none is a PG 1159 star! We conclude that selection effects (e.g. spectral analysis of the brightest CSPNe only) are apparently responsible for the low number of hot, hydrogen-rich PG 1159 counterparts found in previous studies (Méndez et al. 1988, Napiwotzki 1995). Our results are consistent with the ratio of H-rich to H-poor objects of [FORMULA]3 found for white dwarfs and luminous central stars.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999